46 Concrete in Australia Vol 39 No 2
FEATURE: BRIDGES
of excess tie wires, which were left to fall to the base of the
cage. One experience during construction of the headstocks
was that these excess tie wires could be caught between the
steel reinforcement and the cathodic anode ribbons, effectively
causing a short that would render the entire zone inoperable.
As there was only minor contact between the tie and the anode,
continuity readings fluctuated, indicating no open-circuit
conditions as required. ese readings had to be taken seriously,
and the entire cage inspected until the source of the short was
identified and removed. Care had to be taken that all stray tie
wires were removed from the form and cage prior to casting.
Monitoring during and immediately after casting showed
relatively low resistance values between the various circuits,
due to the higher conductivity of fresh concrete, and it was not
possible to confirm electrical isolation at this time. However,
after a few hours of curing, the resistances between the various
circuits shifted back into an acceptable Mega-Ohm range.
7.3 Cable runs and junction boxes
One of the greatest challenges was the wiring of the system
after the headstocks had been completed and the girders lifted
in place. Available space for cabling was sparse and operational
requirements by the client prevented installation of junction
boxes on upstands along the bridge walkway. As a result,
junction boxes had to be located in cast in voids within the
headstocks. Cables were run in the gaps between headstock
and girders. Cable and junction box protection was achieved
by provision of conduits and stainless steel covers.
A total of 117 cables from each span had to be terminated
at the 10 headstock junction boxes along the bridge. is
comprised of 88 cables coming from the girders of each span,
12 cables from each headstock and 17 outgoing cables to the
PSU. Each abutment had an additional small junction box.
Multicore cables were then utilised to connect the terminals
in the junction boxes with the terminals at the monitoring
panels at the power supply unit and the DC output controllers.
It is needless to say that the termination of the cables was
nothing less than testing for the contractor.
External cabling, junction boxes and PSU are expected to
require replacement over the design life of the bridge and thus
have been designed such that replacement is possible.
8.0 COMMISSIONING
Construction of the bridge commenced in 2009 and was
completed in January 2010. Commissioning of the cathodic
prevention system was performed in February 2011 due to
lack of AC power to the site.
e system has now been operational for two years and is
providing full protection to the bridge headstocks and girders.
e reinforcement is receiving between 2 mA/m2 and 6 mA/m2
of protective current which results in a total system output of
20 A and a power consumption of 170 W.
9.0 MONITORING
e client has incorporated a number of practices into its
asset management strategy to ensure the long term success of
this system. ese include monthly inspection of the PSU,
and recording of output values and settings. is inspection
is performed by the client s maintenance staff during
Figure 13. Installation of the anode ribbon system.
Figure 14. Tendons stressed and crossbonded to reinforcement using
tie wires
Figure 15. Anode junction box.